1. Field of the Invention
This invention relates generally to the processing of screens for color cathode ray tubes. More particularly, this invention is directed to improved apparatus and method for forming phosphor and black grille stripes in color cathode ray tubes, and is especially useful for forming extra wide phosphor stripes overlapping black stripes on a shadow mask CRT of the type having a graded pitch mask.
2. Definitions
As used herein the term "pitch axis" means an axis in the screen or mask plane perpendicular to the shadow mask slot lines and to the screen stripe images.
As used herein the term "graded pitch" (mask or screen) means a shadow mask of the slot (or "slit") type or screen of the stripe (or "line") type having a slot/stripe pitch which increases with increasing radial distance from the faceplate center along the pitch axis.
3. Art
The following are being submitted to the Patent and Trademark Office for its evaluation as to their possible relevance to the claimed subject matter. It is believed to be the closest of the art of which applicant(s) is aware, but applicant(s) makes no admission as to the fact of its being "prior art", to its relevance in fact, to its legal sufficiency or to its priority in time, nor does applicant(s) represent that no better art exists.
______________________________________ Item Relevant Disclosure ______________________________________ 1. U.S. Pat. No. 3,947,718 Slot-type shadow mask CRT with van Lent slot pitch increasing off axis 2. "Contoured-Line Screens Slot-type shadow mask CRT with for Color Picture Tubes"; slot pitch increasing off axis IEEE Transactions on Consumer Electronics, Vol. 24, No. 1, 2/78, pgs. 120-125, A.M. Morrell 3. U.S. Pat. No. 3,590,303 - Dot-type shadow mask CRT Coleclough with hole pitch increasing off axis 4. U.S. Pat. No. 3,146,368 - Dot-type black grille shadow Fiore et al mask CRT 5. U.S. Pat. No. 4,070,596 Slot-type black grille shadow Tsuneta et al mask CRT 6. U.S. Pat. No. 3,888,673 - CRT screen exposure method Suzuki et al and apparatus 7. U.S. Pat. No. 3,890,151 - CRT screen exposure method Suzuki et al and apparatus 8. U.S. Pat. No. 3,949,411 - CRT screen exposure method Yonai et al and apparatus 9. U.S. Pat. No. 3,601,018 - CRT screen exposure method Lange and apparatus 10. U.S. Pat. No. 4,078,239 - CRT screen exposure method Prazak et al and apparatus 11. U.S. Pat. No. 3,838,432 - CRT screen exposure method Park and apparatus 12. U.S. Pat. No. 3,667,947 - CRT screen exposure method McKee and apparatus 13. G.B. Pat. No. 1,513,391 - CRT screen exposure method Phillips Electronic & and apparatus Associated Industries Ltd. 14. U.S. Pat. No. 3,856,525 - CRT screen exposure method Inoue and apparatus with oscillating light source for widened phosphor elements ______________________________________
4. Shortcomings of the Prior Art
Among the problems, defects or other drawbacks found in the prior art are the following:
1. Slot-type shadow mask CRT's of the "graded pitch" type with increasing slot pitch off-axis provide increased manufacturing tolerance, but necessitate formation of phosphor strips wider than can be conveniently and reliably fabricated with conventional screen techniques. The following will explain.
It is conventional in tubes having graded pitch masks to seek standard mask transmission profiles from mask center to mask edge (along the pitch axis) by reducing or eliminating the conventional aperture size grading from the center to screen edge--normally provided to give increased manufacturing tolerance.
Having done this, it can be seen that the spacing between the grille openings within which the individual phosphor stripes are deposited is greater than with prior slot mask tubes not having a graded pitch mask and increases with radial distance from the screen center.
It is desirable during deposition of the phosphor stripes to fill not only the grille openings with phosphor material, but to extend the phosphor material onto to the back side of the contiguous black grille areas to an extent that each phosphor stripe touches or nearly touches its neighboring phosphor stripe. This is done to improve the adhesion of the phosphor stripes to the back of the black grille, and to maximize the "leak through" of light emitted by phosphor material overlapping the black grille when bombarded by the electron beam.
The greater guard band or tolerance at the side edges of graded pitch screens, and the widened phosphor stripes required, creates the problem to which this invention is addressed--that is, how to conveniently, reliably and efficiently deposit the necessarily wider phosphor stripes on the black grille of a negative guard band, slot mask, graded pitch type color CRT.
2. In the screening of slot-type shadow masks by conventional methods, the time required to expose photosensitive materials at the edges of the screen is very substantially greater than in the center of the screen (three times, for example). Edge exposure time thus determines the maximum through-put screening rate of CRT faceplates in a CRT factory.
3. Other shortcomings are described in noted prior art U.S. Pat. No. 3,856,525. The solution proposed in the '525 patent suffers, inter alia, from a requirement for apparatus which reciprocates the screen-exposing light source--with the cost, unreliability, maintenance and other such problems customarily attending use of such apparatus.